The ear canal 10, as illustrated in FIG. 1, is generally narrow and tortuous and is approximately 26 millimeters (mm) long from the canal aperture 11 to the tympanic membrane 15 (eardrum). The lateral part 12 is referred to as the cartilaginous canal due to the underlying cartilaginous tissue. Hair and earwax (cerumen) are primarily present in this cartilaginous region 12. The medial part, proximal to the tympanic membrane 15, is rigid and referred to as the bony region 13 due to the underlying bone tissue. A characteristic bend roughly occurring at the bony-cartilaginous junction 8 separates the cartilaginous region 12 and the bony region 13. The dimensions and contours of the ear canal vary significantly among individuals. There is a characteristic “S” shape with a first and second bends generally occurring at the aperture area 11 and junction area 8, respectively (FIG. 2).
Canal dimensions vary significantly along the ear canal and among individuals. Placement of a hearing device substantially inside the ear canal is problematic due to extreme level of miniaturization required and the limited reliability of small parts. Furthermore, access and manipulation of a miniature canal device becomes prohibitive when placed too deeply in the bony region. However, it is desirable to deliver sound deeply, at least into the junction area 8, to achieve electroacoustic advantages including reduction of the acoustic occlusion effect, improved energy efficiency, reduced distortion, reduced receiver vibrations, and improved high frequency response. FIG. 2 shows a top-down view of the ear canal, indicting the narrowness of the contoured ear canal and the challenge of placing and navigating a hearing aid assembly within.
Physiological debris is primarily present in the cartilaginous region 12 of the ear canal, and includes cerumen (earwax), sweat, and oils produced by the various glands underneath the skin in the cartilaginous region. Debris in the ear canal is a major cause of damage to canal hearing devices resulting in clogging of sound ports and frequent repairs. If sounds ports are not protected, debris often flows into the interior, and particularly the microphone causing damage. On the other hand if a sound port is protected by a permanent filter, it will eventually get contaminated by earwax disabling the hearing device.